Anti-reflection film for an optical element consisting of an organic material

ABSTRACT

An anti-reflection film of several layers is described for an optical element consisting of an organic material with photochromic properties. 
     The film according to the invention is characterized by the fact that the mean reflection of the film over the spectral range between 330 nm and 380 nm is less than 4% and the mean reflection over the range between 290 nm and 330 nm is greater than 15%. This reflection behavior serves to prevent the stimulation of the photochromic effect by light with too short a wavelength from reducing the useful life of the photochromic effect. In addition, a suitable choice of the reflection properties means that a deliberate stimulation of photochromic molecules situated in certain steric positions and so influence on the kinetics and color of the photochromic effect can be obtained.

TECHNICAL BACKGROUND

The invention relates to an anti-reflection film for an optical elementconsisting of an organic material with photochromic properties, and inparticular to a spectacle lens of this type.

STATE OF THE ART

In the past a number of proposals have become known to design opticalelements consisting of an organic material and, in particular, spectaclelenses consisting of an organic material with photochromic properties.In this context "photochromic" is to be understood as the property ofthe colour impression and of the darkening of the optical element ofchanging with the amount of illumination, i.e. with the intensity of theradiation.

The photochromic properties of optical elements consisting of an organicmaterial are, as a rule, obtained by introducing organic substances withphotochromic properties, for example, spiro-oxazine compounds, into thematrix of the optical element or by applying them to the opticalelement.

On the other hand, at present most spectacle lenses are provided with ananti-reflection film which will usually consist of several highrefractive index and low refractive index layers. Up until now, thequestion as to which anti-reflection film is best suited to a spectaclelens consisting of an organic material with photochromic properties hasnot been considered in the literature. The same anti-reflection filmsare applied to actually developed spectacle lenses consisting of anorganic material with photochromic properties as are also used forspectacle lenses consisting of an organic material without photochromicproperties.

SUMMARY OF THE INVENTION

It has now been recognised in accordance with this invention that theproperties of spectacle lenses consisting of an organic material intowhich photochromic substances have been introduced or onto whichphotochromic substances have been applied, can be influenced by thesuitable choice of an anti-reflection film.

For this reason, it is the object of this invention to provide ananti-reflection film on an optical element consisting of an organicmaterial with photochromic properties.

In accordance with this invention it has been recognised that ananti-reflection film of several layers for an optical element consistingof an organic material with photochromic properties must possess thefollowing characteristics:

1. In the wavelength range between 330 nm and 380 nm the reflectionshould be as low as possible to ensure a sufficient stimulation of thephotochromic substance or, in the case of several substances having beenintroduced, of the different substances.

2. In the wavelength range below 330 nm the reflection should increasesteeply as radiation in this wavelength range, which also stimulates thephotochromic effect, will--as has also been recognised in accordancewith the invention--reduced the life of the photochromic effect.

3. In the wavelength range below 290 nm the reflection properties of theanti-reflection film no longer play a decisive role as the absorptionproperties of the atmosphere prevent such radiation from stimulating andso perhaps damaging the organic substances to any noticeable extent.

This means that the anti-reflection film provided in accordance withthis invention differs from the films used up to now for both spectaclelenses in plastic without photochromic properties and for such withphotochromic properties in that it has defined reflection properties notonly in the range of visible light from 380 nm to 780 nm but also in therange from 290 nm to 380 nm. No consideration has been given up to nowto the curve behaviour below 380 nm.

The film in accordance with the invention provided on an optical elementconsisting of an organic material and, in particular, for a spectaclelens with photochromic properties has the following advantages:

It ensures a sufficient stimulation of the photochromic effect independence on the pertaining amount of illumination. As a result, whenthe anti-reflection film according to the invention is used, thekinetics of the photochromic effect is better than when conventionalfilms are used.

It prevents the life of the photochromic effect from being reduced bythe stimulation of the photochromic effect by light with too short awavelength.

In addition, further unexpected advantages are produced by the use ofthe photochromic film according to the invention:

Due to the different steric positions of the photochromic molecules inthe plastic matrix, these molecules show different absorption andkinetic properties. With a suitable choice of the reflection propertiesof the film a deliberate stimulation of photochromic materials situatedin certain steric positions and so influence on the kinetics and colourof the photochromic effect can be obtained.

If at least two different photochromic substances which have differentabsorption spectra are integrated in the matrix of the optical element,which, for example, may consist of organic material, then a considerableinfluence on the colour of the photochromic effect can be obtained bymeans of the design of the reflection properties in the range between290 nm and 380 nm.

As has further been recognised in accordance with the invention, theproperties according to the invention of the anti-reflection film for anoptical element consisting of an organic material with photochromicproperties can only be fulfilled with a layer system of more than threelayers. Preferred version forms of this layer system are hereinafterdescribed. With a film which has 5 layers consisting alternately ofsilicon dioxide and a material with a high refractive index, thebehaviour provided in accordance with the invention between 290 nm and380 nm can only be obtained satisfactorily, if the high-index materialhas a refractive index n_(e) which is at least 2.0.

BRIEF DESCRIPTION OF THE DRAWING

The invention described in more detail below by means of version examplewith reference to the drawing in which

FIG. 1 shows a film in accordance with the invention with 5 layers and

FIGS. 2-5 show reflection curves of different version examples.

METHOD OF PERFORMING THE INVENTION

FIG. 1 shows a film in accordance with the invention with 5 layers ofwhich the first layer 1 applied directly to the optical element 0consists of SiO₂, the second layer 2 of a high-index metal oxide, thethird layer 3 of SiO₂, the fourth layer 4 again of a high-index metaloxide and the fifth layer 5 consists of SiO₂.

Furthermore, it is also possible to split up layer 1 into three partiallayers 1', 1", 1'" consisting of SiO₂, a high-index metal oxide andSiO₂.

In the following Table 1, the design of the layer system with 5 layersis shown for different high-index materials (HIM) as is the refractiveindex n_(e) of the high-index material, i.e. the refractive index forlight with a wavelength of 546.1 nm. It is assumed here that thesubstrate 0 has a refractive index of (about) 1.502.

                  TABLE 1                                                         ______________________________________                                        HIM         Tb.sub.2 O.sub.3                                                                      Nd.sub.2 O.sub.3                                                                           Ti.sub.2 O.sub.3                             Layer/n.sub.e                                                                             2.00    2.10         2.33                                         ______________________________________                                        1 (SiO.sub.2)                                                                             175     175          180  nm                                      2 (HIM)     17.3    14.2         12.9 nm                                      3 (SiO.sub.2)                                                                             36.4    37.1         35.3 nm                                      4 (HIM)     121.3   123.8        120.8                                                                              nm                                      5 (SiO.sub.2)                                                                             87.5    87.5         91.2 nm                                      ______________________________________                                    

In Table 2 a version example is shown for a layer system with 7 layers.

                  TABLE 2                                                         ______________________________________                                        HIM:     Ti.sub.2 O.sub.3 ; n.sub.e = 2.23                                    ______________________________________                                        1' (SiO.sub.2) =                                                                       50; 1" (HIM) = 5.2;                                                                            1'" (SiO.sub.2) = 112.3                             2 (HIM) =                                                                              10.9; 3 (SiO.sub.2) = 37.3;                                                                    4 (HIM) = 115.6                                     5 (SiO.sub.2) =                                                                        78.4                                                                 ______________________________________                                    

FIGS. 2 to 5 show the reflection factors r(λ) (in %) of the layersystems shown in Table 1 or 2 as a function of the wavelength λ of theincident light. As a common property of all layer systems it can clearlybe seen that the maximum r(λ) value of the reflection does not excees avalue of 6% in the range between 330 nm and 380 nm while the meanreflection r(λ) calculated by

    r(λ)=∫r(λ)dλ/∫dλ

is less than 3% over this wavelength range. In the range between 290 nmand 330 nm the maximum value of the reflection exceeds the value of 25%with the mean reflection over this range being greater than 15%. Thefall below 290 nm, which occurs with some reflection curves, is withoutmeaning as radiation plays no role in this wavelength range due to theabsorption properties of the atmosphere.

The invention has been described by means of examples above. However, itgoes without saying that within the central idea according thisinvention, the life, the kinetics and, when differently colouringphotochromic substances are used, the colour impression can beinfluenced by a suitable design of the anti-reflection film and that themost varied modifications are possible.

For example, the layer system in accordance with the invention can notonly be used for photochromic optical elements with a refractive indexof around 1.5 but also for elements with a higher refractive index. Itis also possible to use the most varied materials with other refractiveindices instead of the high-index materials shown in the examples aslong as the other refractive indices are greater than or equal to 2.0.Different high-index materials can also be used on one layer system.

However, in each case it is of particular advantage to keep to thecondition for the thickness ratio given according to this invention. Butit also goes without saying that the relationship given need only be metwithin certain, material-dependent tolerances.

In addition, optical elements can also be coated which do not consist ofan organic material as long as the photochromic substances they containhave similar properties to those given above.

We claim:
 1. An anti-reflection film of several alternative inorganic layers on an optical element consisting of an organic material with photochromic propertiescharacterized by the fact that the mean reflection of the film over the range between 330 nm and 380 nm is less than 4%; the mean reflection over the range between 290 nm and 330 nm is greater than 15%; and the film has at least five layers which consist alternately of silicon dioxide and a metal oxide material with a high refractive index of n≧2.0, and of which the first layer applied directly to the optical element consists of silicon dioxide and has a thickness of about λ/2, the thicknesses of the second, third and fourth layers behave as

    1:2Y:20Z/3

where

    Y=X+1.5(n.sub.e -1.8); Z=X±0.2;

    X=1.05+(n.sub.e -2)1/(4-n.sub.e)

and the sum of the thicknesses of the second to fourth layer is also about λ/2 with the fifth layer being a λ/4 layer (λ=350 nm).
 2. A film according to claim 1,characterised by the fact that the first layer is split up into three partial layers consisting of silicon dioxide, the high-index material and again silicon dioxide, and that the thickness of the middle high-index layer is λ/100 to λ/50.
 3. A film according to claim 2,characterised by the fact that the high-index material is neodymium oxide, titanium oxide or terbium oxide.
 4. A film according to claim 1, characterized by the fact that the high-index material is neodymium oxide, titanium oxide or terbium oxide.
 5. An anti-reflection film of several alternative inorganic layers on an optical element consisting of inorganic material with photochromic propertiescharacterized by the fact that the mean reflection of the film over the range between 330 nm and 380 nm is less than 4%; the mean reflection over the range between 290 nm and 330 nm is greater than 15%; the optical element is a spectacle lens; and the film is composed of at least three alternative layers, a first layer being silicon dioxide, a second layer being a metal oxide having a refractive index of n≧2.0 and a third layer being silicon dioxide.
 6. A film according to claim 5, wherein the metal oxide is neodymium oxide, titanium oxide or terbium oxide. 